This project mainly concerned the development of novel engineering approaches
to optimise the physical properties of the polymer composites with a low loading
of carbon nanotubes (CNTs). It was additionally discovered that graphite oxide
nanoplatelets (GONPs) can be a strong and affordable substitute for the CNTs in
the polymer composites.
Colloidal physics and coating methods were applied to fabricate semi-conductive
CNT/polymer composites with low percolation threshold. Polyurethane (PU) latex
and ultra high molecular weight polyethylene (UHWMPE) powder were used as
hosting matrix in the colloidal physics method and coating method, respectively.
In the colloidal physics method, the percolation threshold was found to be around
O.5wt% MWCNTs and the electrical conductivity of the composites was improved
by more than four orders of magnitude with the addition of I wt % multi-walled
carbon nanotubes (MWCNTs). The study of rheological behaviour revealed that
the addition of the MWCNTs led to the increase in the viscosity of the PU
dispersion. In the coating method, the scanning electron microscopy (SEM)
images confirmed the strong adhesion of the nanotubes on the surface of the
powders. Sheet samples were prepared using compression moulding for electrical
test. The percolation threshold for the powders with the size of 60)lm was around
I wt% MWCNTs and the percolation threshold for the powders with the size of
100)lm was around 0.5wt% MWCNTs.
A novel route was revealed to reduce the interfacial phonon scattering that is
considered as the bottleneck for CNTs to highly improve the thermal conductivity
of CNT/polymer composites. Semicrystalline PU dispersions were used as latex
host to accommodate the MWCNTs following the colloidal physics method. The
thermal conductivity increased from 0.15 Wm-'K-' to 0.47 Wm-'K", by -210%, as
the addition of the MWCNTs increased to 3wt%. The morphology of the
composites suggested that the continuous nanotube-rich phase existing in the
interstitial space among the latex particles and the crystaIIites nucleated at the
nanotube-polymer interface were the main factors for the effective reduction of
interfacial phonon scattering.
The optimisation of the crystalline layer around CNTs was studied based on the
MWCNT/polycapro!actone (PCL) composites using differential scanning
calorimetry (DSC). The study of the non-isothermal crystaIlisation showed that
crystaIIisation temperature (Tc) increased with increasing incorporation of the
nanotubes, and melting temperature (T m) and heat of fusion (ilHm) was almost
unchanged. The incorporation of 2wt% nanotubes resulted in the biggest increase
of the T c to be -11 QC. The study of the isothermal crystaIIisation showed the
temperature, 14 DC higher than the Tc. was appropriate one to optimise the
crystaIIine layer in the composite melts. It was revealed that the incorporation of
0.1 wt% nanotubes significantly affected the rate of crystal growth and crystalline
morphology. For more incorporation of the nanotubes, the rate of crystal growth
and crystaIIine morphology was less affected. The improvement in the Young's
modulus of the composite with the thermal treatment confirmed the contribution of
the crystalline layer to the load transfer across the non-covalent interface between
the nanotube and polymer matrix.
The preparation of the exfoliated GONPs in DMF was revealed. With this method
in hand, two kinds of polymers including semi-crystaIline PCL and amorphous PU
were selected to be incorporated with the GONPs using the solution method. It
was found that the GONPs showed strong nucleating ability in the PCL matrix.
The thermal treatment under the "14QC" rule could create an optimised crystalline
layer on the surface of the GONPs from the composite melts. The bigger increase
in the Young's modulus of the treated GONPIPCL composites confirmed that the
crystaIIine layer nucleated on the surface of the GONPs could act as a
non-covalent interface between the GONPs and PCL matrix. The significant
reinforcement of the PU using GONPs was also disclosed. Morphologic studies
showed thai, due to the formation of chemical bonding, strong interaction occurred
between the GONPs and the hard segment ofthe PU, which allowed effective load
transfer. The GONPs can prevent the formation of crystalline hard segments due to
their two-dimensional structure. With the incorporation of 4.4wt% graphite oxide
nanoplatelets, the Young's modulus and hardness of the PU were significantly
increased by -900% and -327%, respectively. The resultant high anti-scratch
property pointed to the promising application of these composite materials in
surface coating.

Description:

A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University